Upgrade Tailwind CSS from 3 to 4 with CSS-first config migration

Migrate tailwind.config.js to @theme in CSS, replace PostCSS plugins
with @tailwindcss/postcss, add @reference to 12 Vue scoped styles using
@apply, and remove autoprefixer (now built into Tailwind 4).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/Discover/Conformance/ConformanceSidebar/ActSeqDrag.vue

@@ -130,6 +130,7 @@ export default {
 }
 </script>
 <style scoped>
+@reference "../../../../assets/tailwind.css";
 .ghostSelected {
   @apply bg-primary/20
 }